We review energy and economic requirements, available technologies, and limiting factors for direct air capture systems. As the concentration of carbon dioxide in the Earth's atmosphere continues to increase, discussion about various methods to bring this gas under control intensify. Current data from the US National Oceanic and Atmospheric Administration's Earth System Research Laboratory: Global Monitoring Division (NOAA/ESRL) shows atmospheric carbon dioxide concentrations teasing the 400 ppm by volume threshold in 2015 (Dlugokencky & Tans, 2015). The climate goal noted in the Copenhagen Accord, and discussed internationally since, is “to hold the increase in global temperature below 2 degrees Celsius” (UNFCCC, 2010, p. 5, 2015). To obtain this goal, various pathways have been suggested by the IPCC which incorporate different scenarios utilizing varying combinations of methods and technologies (IPCC, 2014). While ideally atmospheric carbon dioxide would be reduced through a decrease in consumption and introduction of new technologies that eliminate emissions, reduce emissions, or capture emissions at their source, removing atmospheric carbon dioxide after release into the atmosphere for sequestration is also an option. Direct air capture (DAC) is one of several proposed technologies intended to remove carbon dioxide directly from the atmosphere, regardless of its source. Since the atmosphere effectively disperses CO2 emissions from both large and small sources quickly and evenly, a continuous supply of CO2 laden air would naturally be delivered to any DAC site, with no local buildup of CO2 depleted air likely (Goeppert, Czaun, Surya Prakash, & Olah, 2012, p. 7837). Debate about the efficiency and practicality of this technology has been heated, but as atmospheric carbon continues to rise the pathways necessary to achieve the 2 degree goal become increasingly dependent on achieving negative carbon emissions by the second half of this century (IPCC, 2014, p. 490). The success of technologies such as DAC become ever more important
